http://www.nissanpartszone.com/Page_Product/PartDetail.aspx?ModelName=720+Pickup&ModelYear=1981&ModelSeries=720&FileType=236&SelectConditions=(1%3dK%2fCAB%3b2%3dSD22%3b3%3dDX%3b4%3dF5%3b5%3dLONG%3b6%3d2WD%3b)&ShowConditions=(1%3dK%2fCAB%3b2%3dSD22%3b)&seeDesc=False&goBack=True&SectionNo=B&ComponentsNo=165&ComponentsIndex=3
Its expensive but you can buy the airbox on this website and the piping.....My 82 king diesel had the same issue, guess the boxes were not very sturdy...instead of replacing the airbox I simply adapted a K&N filter to it and it works great. Figured without the air box i could gain a 1/2 mile per hour lol